I've been using GHK-Cu, almost through my first 2 month cycle. Can't really speak to the skin tightening aspects of it yet as I had already lost about 55 lbs when I found out about it, now at about 72.
BUT, one thing I have noticed: I've got a genetic disorder called Ehlers Danlos Syndrome, that affects connective tissue and collagen in the body. My finger nails have always been extremely brittle. They split super easily, I keep them cut as close to the quick as I can, because If I don't, they will spilt vertically INTO the nail bed and that really sucks. Here's the thing, starting about 2 weeks into the cycle, I noticed my nails were stronger. I had forgotten to clip them and they weren't splitting! Since then I've only had one split, when I banged my finger on a hard surface.
Now I still keep my nails cut pretty short, and they aren't as strong as a non-EDS person, but they are WAY better than prior to starting the GHK-Cu.
So that's my experience thus far.